Human abilities are remarkably responsive to the effects of practice. Studies of the effects of practice, in both laboratory and natural settings, have documented the extent to which humans can obtain exceptionally high levels of performance in tasks ranging from motor to cognitive. Within the past two decades, a rapidly-expanding literature on perceptual learning has documented the range of tasks in which practice on basic perceptual judgments can produce dramatic improvements in performance. These studies suggest that even elementary aspects of human information processing can be substantially modified and, by extension, that early sensory levels may be highly plastic. The specific focus of this proposal is on visual tasks that involve stimulus characteristics manipulated at levels at or near observers'detection thresholds. While specific tasks may require categorization, identification, or discrimination judgments, the need to accurately distinguish between the presence and absence of a characteristic is a fundamental aspect of the perceptual abilities that are of interest;an example would be the extent to which a skilled radiologist is able to both detect and identify a tumor in an X-ray image. The available evidence on these types of skills suggests three hypotheses: (a) The acquisition and expression of perceptual expertise relies on multiple levels of encoded information, (b) All of these codes are available simultaneously, (c) The time-course of learning involves an incremental strengthening of codes at increasingly lower levels of the visual system. Tests of these hypotheses will involve the collection of both behavioral and neurophysiological (EEG) data, and the development of a computational modeling approach capable of linking these two types of data. Relevance: The explicit concern with perceptual abilities that require both detection and identification holds the potential for translational application in domains in which observers must acquire the ability to reliably distinguish between the presence and absence of some visual attribute, and to reliably categorize or name an object based on that attribute when the information available to observers may be extremely limited. Examples include behavioral interventions for visual impairments (e.g., amblyopia), the training of police sharp-shooters, or the training of security screeners.